The present invention is directed to axial fans and fan drive systems for evaporative, air, water, and hybrid wet/dry cooling equipment. Common applications for evaporative cooling equipment, such as cooling towers, include providing cooled process medium for heating, ventilation, air conditioning, and refrigeration (“HVACR”), manufacturing, industrial processes, and electric power generation. In operation, the cooling towers serve to transfer heat from the process medium into the surrounding environment. Similarly, common applications for non-evaporative cooling equipment, such as condensers, include providing cooled process medium for HVACR, manufacturing, industrial processes and electric power generation. Finally, common applications for hybrid cooling equipment, such as wet/dry closed circuit coolers, include providing cooled process medium for HVACR, manufacturing, industrial processes, and electric power generation. Generally speaking, as is generally known in the art, WET/DRY cooling equipment serve to transfer heat from the process medium into the surrounding environment. Such WET/DRY cooling equipment may be a standalone piece of equipment or a part of a larger “packaged” piece of HVACR or industrial equipment.
In an open circuit cooling tower, the process fluid to be cooled is delivered to the cooling tower and is typically distributed by a series of nozzles that atomize the process fluid over a heat transfer medium located inside the heat exchanger section, commonly referred to as a “fill.” The fill facilitates heat transfer by promoting evaporation through commingling the process fluid with dry, outside air. The fill provides a large surface area and facilitates contact between the process fluid and the dry, unsaturated airstream supplied by a fan within the cooling tower. As the process fluid droplets pass through the fill, heat is transferred to the atmosphere through the discharge airstream of the cooling tower. A portion of the process fluid is lost through the endothermic process of evaporation, leaving the remaining process fluid at a lower temperature than it was before it entered the cooling tower. The cooled process fluid is collected in a collection basin at the bottom of the cooling tower and then withdrawn therefrom.
Closed-circuit cooling towers, also known as fluid coolers, have similar functionality, with a difference being that the process fluid is contained within one or more heat transfer coils and not directly exposed to the surrounding environment. Water stored in the collection basin of the unit is typically sprayed over the coil(s) to promote heat transfer from the liquid to the make-up water, while at the same time promoting the endothermic process of evaporation. The end result is the process fluid within the coil is cooled through evaporation of spray water on the outside surface of the coil, and to a lesser degree, heat is transferred through the temperature gradient between the spray water/intake air temperature and the coil when atmospheric conditions allow.
Evaporative condensers are nearly identical to a closed-circuit cooling tower, or fluid cooler, except for the process medium. In the case of an evaporative condenser, the process medium is a refrigerant delivered directly from the evaporator of an HVACR machine. The evaporative condensers are typically used in the refrigeration industry, cold storage, ice skating rinks, cryogenics, and so forth. Hybrid versions of closed-circuit cooling towers employ the addition of fins to the coil circuits similar in design to those employed on air cooled condensers and heat exchangers. Where atmospheric conditions and/or systems load conditions allow, the fluid cooler is switched from the conventional evaporative, a.k.a “wet operation,” cooling mode to an air-cooled, a.k.a. “dry operation,” by switching off the spray water pump. This effectively changes the machine from a closed-circuit cooling tower into an air-cooled condenser/heat exchanger. The purpose of these hybrid cooling units is to save water and energy by arresting the evaporation of water and the elimination of the energy required to operate the spray water pump when atmospheric conditions and system load conditions allow.
Non-evaporative condensers and coolers have similar functionality to closed-circuit cooling towers with the difference being that they rely solely on heat transfer through direct and/or indirect contact of the process medium and the heat exchanger surface with outside air. Non-evaporative condensers and coolers have similar construction and component arrangements to closed-circuit cooling towers with a difference being that they omit components associated with evaporative cooling process, such as, but not limited to, spray water pump, distribution systems, drift eliminators, and collection basins. Air-cooled condensers and coolers use heat exchangers of the “Liquid to Air” or “Gas to Air” variety, while and water-cooled condensers and coolers use heat exchangers of the “Liquid to Liquid” or “Gas to Liquid” variety, which is similar in design and construction to those employed in closed-circuit cooling towers.
In operation, airflow through WET/DRY cooling equipment is typically facilitated by a fan in combination with an intake air conduit and an exhaust air conduit, which are provided for each heat transfer section, unit, or cell, of the equipment. In induced-draft equipment, the fan is typically mounted near the exhaust of the unit and used to draw air from the intake through the interior of the unit and across the heat exchange surface located inside the heat exchanger section. In forced-draft equipment, the fan is typically mounted near the intake and pushes the air through the interior of the cooling unit, across the heat exchange surface located inside the heat exchanger section and out via the exhaust.
Several considerations are present during the installation and design WET/DRY cooling systems, including airflow, sound output, space requirements, energy requirements, and vibration transmission. It is desirable to minimize noise emitted by operation of the fan, the energy consumed by the fan drive system, and the vibrations emitted by the fan drive system. However, minimizing these negative attributes requires reducing the rotational speed of the fans, which limits the heat exchange capacity of a given unit design by falling below the required minimum airflow and static pressure. Independent of minimizing negative attributes of the conventional axial fan systems currently in use, it is also desirable to employ a fan arrangement with a higher overall efficiency that can generate an increased amount of airflow and static pressure at a given energy input value. Such a fan arrangement would increase the thermal capacity ratings, and energy efficiencies of existing WET/DRY cooling equipment designs, while at the same time increasing the energy efficiency of the entire heat transfer system in which they are installed.
As disclosed herein, one solution to minimize the negative attributes and/or increase thermal capacity ratings and energy efficiencies of WET/DRY cooling equipment is the use of a contra-rotating, multi-stage fan arrangement. A contra-rotating, multi-stage fan arrangement is capable of meeting minimum airflow and static pressure requirements at rotational speeds that are lower than that of currently-employed axial fan systems. A contra-rotating, multi-stage fan arrangement is also capable increased airflow and static pressure at a given energy input than that of currently-employed axial fan systems.